1. Field of the Invention
The invention relates to a speed control apparatus for maintaining a brushless direct current motor at a predetermined speed, and, more particularly, to a speed control apparatus for a brushless direct current motor of the type having Hall elements arranged upon detection of the angular position of a permanent magnet rotor to provide Hall voltages induced therein. The Hall elements control a transistor drive circuit for successive energization of the field coils. There are provide, in the apparatus, a signal generator for generating a reference signal corresponding to a desired speed, detecting means for detecting the output signal from said generator and the Hall voltage difference from said Hall elements, and supply means for supplying a counter-EMF developed across a field coil of the motor during an interval of deenergization thereof in reversed relationship to the drive current when the output signal from said detecting means is larger than a predetermined value. The supply means are arranged so that when the output signal from said detecting means exceeds the predetermined value, a field coil to be energized is supplied with a drive current from said drive circuit and with the counter-EMF from said supply means.
2. Description of the Prior Art
A conventional speed control apparatus for maintaining a brushless D.C. motor at a predetermined speed makes use of Hall elements arranged upon detection of the angular position of a permanent magnet rotor to provide voltages induced therein which control a transistor drive circuit for successive energization of the field coils. However, such a conventional apparatus has a drawback that when the speed of the D.C. motor exceeds a desired speed due to fluctuation in the load being driven, a considerably long time interval is required until the motor regains the reference speed.